Structural Geology and Rock Mechanics

Structural Geology

deals with the geometric relationships of rocks and geologic features in general

Structural Geology

studies the 3D geometry from micro to macro scale of rocks to explain the deformation processes the rocks experienced since their origination

Structural Geology

it introduces the physical side of Geological Sciences and emphasizes Geometry, Motion, and Mechanics

Geometry, Motion, and Mechanics

three things that are emphasized by Structural Geology

Geometry

shape, orientation, position, size, etc.)

Motion

beginning and ending positions and paths of particles and bodies— deformation or change in geometry)

Mechanics

explanations of why the geometry and motion are as they are)

Rock Mechanics

theoretical and applied science of the mechanical behaviour of rock

Rock Mechanics

branch of mechanics concerned with the response of rock to the force fields of its physical environment

Structural rock mechanics and Comminution

branches of Rock Mechanics

Structural rock mechanics

concerned with the stability of engineering structures in which the material is predominantly rock.

Comminution

which is concerned with the reduction of rock to small fragments by the application of external forces as in drilling, blasting, cutting and grinding

Global

A scale covering almost the entire world

Regional or Provincial

Roughly definable; generally corresponds to a physiographic province. Taurus Mountains, Himalayan Plato

Macroscopic or Map Scale

Larger than an area one can see from a particular point on the ground

Mesoscopic

An area visible from a particular point on the ground (outcrop to hand sample)

Submicroscopic

Visible the with help of an advanced microscopic device like TEM (Transmission electron microscopy) or SEM (scanning electron microscope)

Penetrative

Characterizes the entire body of the rock

Non penetrative

Does not characterize the entire body of the rock (ex. a part of the body)

Primary structures

structures that develop during the formation of the rock

Primary structures

represent the local conditions of the environment within which the rock forms (Davis & Reynolds, 1996)

Primary structures

Ex. Bedding, ripple mark or cross bedding in sedimentary rocks

Secondary structures

structures that develop in sedimentary or igneous rocks after lithification

Secondary structures

in metamorphic rocks during or after their formation

Secondary structures

Fundamental secondary structures are joints and shear fractures; faults, folds, cleavage, foliations, lineations, shear zones (Davis & Reynolds, 1996)

Trend

The direction of a horizontal line specified by its bearing or azimuth.

Bearing

The horizontal angle measured east or west from the true north or south.

Azimuth

The horizontal angle measured clockwise from the true north.

Strike

the trend of a horizontal line on an inclined plane. It is marked by the line of the intersection with a horizontal plane. (Davis & Reynolds, 1996).

Inclination

The vertical angle, measured downward from the horizontal to a sloping plane or line.

Fracture

a local separation or discontinuity plane in a geologic formation, such as joints or faults

Fracture

very narrow zones, often thought of as surfaces, associated with discontinuities in displacement and mechanical properties (strength or stiffness)

Fracture

commonly referred to as cracks in material science and rock mechanics

shear fracture or slip surface

a fracture along which the relative movement is parallel to the fracture

fault

more commonly restricted to discontinuities with larger offset

fault

Distinct fracture surfaces along which rocks have been offset by movement parallel to the fracture surface.

slip surface

used for fractures with fracture parallel movements regardless of the amount of displacement and is consistent with the traditional use of the term fault

Extension fractures

are fractures that show extension perpendicular to the walls

Extension fractures

are typical for deformation under low or no confining pressure, and form at low differential stress

Extension fractures

the most common type of extension fracture at or near the surface of the Earth and involve very small strains

Tensile Fractures

are extension fractures that form under conditions where at least one of the stress axes is tensile

Joints

have little or no macroscopically detectable displacement, but close examination reveals that most joints have a minute extensional displacement across the joint surfaces

Joints

separation in rock where the amount of separation is not greater than the displacement associated with the opening of the fracture.

Fissures

When filled with air or fluid; more open than joints, and are characteristic of the uppermost few hundred meters of the solid crust

Veins

Mineral filled extension fractures

Dikes

magma filled fractures

Faults

Distinct fracture surfaces along which rocks have been offset by movement parallel to the fracture surface.

Folds

are planar surfaces that are curved or bent due to external forces.

Anticline and Syncline

two types of folds

anticline

is a type of fold that is an arch like shape and has its oldest beds at its core

syncline

is a fold with younger layers closer to the center of the structure

Foliation

is any penetrative planar fabric or layering in a rock. (Marshak & Mitra, 1988)

Lineation

Is a preferred linear alignment of elements in rocks. (Davis & Reynolds, 1996)

Shear zone

Is a general term for a relatively narrow zone with subparallel boundaries in which rocks are more highly deformed than rocks adjacent to the zone. (Marshak & Mitra, 1988)

Geologic contact

dashed on the coast where bedrock is absent, and across ponds and lakes

Strike slip fault

inferred where line is dotted

Thrust fault

teeth on the upper plate, inferred where line is dashed

Fault

direction of movement uncertain

Normal fault

inferred where line is dotted

SEISMIC VELOCITY

Elastic energy propagates through the earth in different ways and at different speeds.

SEISMIC VELOCITY

Each wave has a distinct particle motion, elastic deformation and speed of propagation.

SEISMIC VELOCITY

The speed of propagation depends upon the elastic properties and the density of the medium.

ELASTIC PROPERTIES

used to define physical deformation experienced in response to an applied mechanical force

ELASTIC PROPERTIES

energy from initial elastic deformations are transferred to adjacent materials

Energy transfer

continues as deformation propagates away from the source.

Seismic waves

are used to describe the speed, direction and location of elastic deformations as they propagate through materials.

ELASTIC DEFORMATION

implies that material returns to original volume and shape once the applied force is removed; therefore conserves energy.

ELASTIC DEFORMATION

Change in volume and/or shape occurs when a mechanical force is applied.

ELASTIC DEFORMATION

This is commonly described in terms of: a)Stress b)Strain

STRESS

is defined as the internal reaction of the body to the external force applied to a body over its cross

Normal, Compressive, Tensile, Shear

Main types of stress

STRAIN

is the deformation of a material under the influence of an applied stress.

ELASTIC STRAIN

For relatively small strains, body will recover its original shape and volume when stress is removed

PLASTIC STRAIN

For large strain, the body deforms continuously and may even fracture

PLASTIC STRAIN

this type permanently alters the shape of the body.

Material Elastic Properties

Seismic waves cause the earth to undergo linear elastic deformation

Material Elastic Properties

an application of Hooke’s Law assumes a linear relationship between stress and strain

LAMÉ PARAMETERS

These are two material dependent quantities denoted byλand μ that arise in strain stress relationships (shear modulus and elastic modulus)

μ or G

Shear modulus

λ

Elastic modulus, Young’s modulus, Eor Bulk modulus, K

Shear Modulus

The parameter defines resistance of object to shear.

Shear Modulus

Sometimes referred to as the modulus of rigidity.

Shear Modulus

When a block of material is subjected to a shear stress, it experiences a shear strain

ELASTIC MODULUS

Also referred to as the modulus of incompressibility

ELASTIC MODULUS

it is the resistance of a material to elastic compression.

Bulk modulus, K

The ratio of volumetric stress to the volumetric strain.

Young’s modulus, Y or E

The ratio of linear (longitudinal) stress to the linear (longitudinal) strain

Poisson’s Ratio

The ratio of transverse strain to longitudinal strain due to a longitudinal stress.

Poisson’s Ratio

The decrease in length could be compensated for by decrease in pore space.

SEISMIC WAVE VELOCITIES

They define the speed at which various elastic deformations propagate through materials.

Rocks and other materials

can be characterized by their elastic properties

SEISMIC WAVE VELOCITIES

represent a more practical set of physical properties for seismic methods.

Body waves

Seismic waves that move through the interior of earth.

Pressure waves (Pwaves)

Also known as Primary waves

Pressure waves (Pwaves)

are compressional waves in which particle motion is in the direction of the wave propagation

Pressure waves (Pwaves)

travel faster than other seismic waves, hence they are the first signal from earthquake to arrive at any affected location or at a seismograph

Pressure waves (Pwaves)

may be transmitted through gases, liquids, or solids

Shear waves (Swaves)

also known as Secondary waves or elastic Swave.

Shear waves (Swaves)

are waves in which the particle motion is perpendicular to the direction of wave propagation

Surface waves

Waves that travel near the earth’s surface.